In the related art, e.g., in a membrane bioreactor system, a submerged membrane separator is submerged in a reaction tank where activated sludge treatment is performed on sewage or the like. In such a membrane separator, multiple membrane cartridges of an organic flat-membrane type are arranged in parallel at predetermined intervals in the main-unit casing.
As shown in FIGS. 9 and 10, a membrane cartridge 10 includes a rectangular filtration plate 11 extended in vertical direction A and filtration membranes 12 provided on both surfaces of the filtration plate 11.
Further, channel grooves 13 are formed on both surfaces of the filtration plate 11. A permeated liquid having passed through the filtration membranes 12 flows through the channel grooves 13. The channel grooves 13 are extended in the vertical direction A (longitudinal direction) and are arranged in parallel in width direction B of the filtration plate 11.
On the upper end of the filtration plate 11, a liquid collection part 14 for collecting the permeated liquid in the channel grooves 13 is formed. The liquid collection part 14 penetrates both filtration plate surfaces of the filtration plate 11 and the upper ends of the channel grooves 13 communicate with the liquid collection part 14. Moreover, the channel grooves 13 and the liquid collection part 14 constitute a permeated liquid channel 17. On the upper edge of the filtration plate 11, a permeated liquid outlet nozzle 15 is provided through which the permeated liquid collected in the liquid collection part 14 is discharged out of the membrane cartridge 10.
With this configuration, when the membrane cartridge 10 is used for solid-liquid separation, a suction pressure (negative pressure) is applied to the permeated liquid outlet nozzle 15 by using a suction pump, so that the suction pressure is applied to the permeated liquid channel 17 (i.e., the channel grooves 13 and the liquid collection part 14) through the permeated liquid outlet nozzle 15 and a tank liquid mixture 18 (liquid to be treated) is filtered through the filtration membranes 12. At this point, the filtered liquid having passed through the filtration membranes 12 flows into the liquid collection part 14 through the channel grooves 13 and the liquid collected in the liquid collection part 14 is discharged out of the membrane cartridge 10 from the liquid collection part 14 through the permeated liquid outlet nozzle 15.
For example, the published unexamined patent application of patent literature 1 describes the membrane cartridge 10 in which the channel grooves 13 and the liquid collection part 14 are formed thus on the filtration plate 11.
When a suction pressure is applied to the permeated liquid channel 17 of the membrane cartridge 10, a pressure loss to the permeated liquid outlet nozzle 15 increases toward the lower part of the membrane cartridge 10. FIG. 11 shows suction pressure distribution on the surface of the filtration plate 11. In FIG. 11, constant-pressure lines 19a to 19e each indicate positions having equal pressures. The constant-pressure lines 19a to 19e indicate lower pressures toward the lower part of the filtration plate 11. In the pressure distribution, the constant-pressure lines 19a to 19e are extended downward directly under the permeated liquid outlet nozzle 15 and the suction pressure decreases away from the permeated liquid outlet nozzle 15. Further, in the pressure distribution in the width direction B, the suction pressure decreases away from beneath the permeated liquid outlet nozzle 15 in the width direction B.